The present invention relates to materials which promote the removal, by adsorption, of the nitrogen oxides (NO and NO2, commonly called NOx) present in a gaseous mixture which can contain a superstoichiometric proportion of oxidizing compounds, more particularly oxygen, said materials not being poisoned by the sulfur products found in these gases. The invention applies to the removal of the nitrogen oxides (NOx) present in the engine exhaust gases from automotive vehicles and very particularly from diesel vehicles.
The high toxicity of nitrogen oxides and their role in the formation of acid rain and tropospheric ozone have resulted in the imposition of strict standards limiting the discharges of these compounds. To meet these standards, it is generally necessary to remove at least part of these oxides present in the exhaust gases from automotive or stationary engines and from turbines.
It is possible to envisage removing the nitrogen oxides by thermal or, preferably, catalytic decomposition, but the high temperatures demanded by this reaction are incompatible with those of the exhaust gases. Catalytic reduction of the nitrogen oxides to nitrogen can only be effected by using the reducing agents which are present, albeit in small amounts, in the exhaust gas (CO, H2, unburnt hydrocarbons or hydrocarbons imperfectly combusted in the engine) or by injecting complementary reducing compounds upstream of the catalyst. These reducing agents are hydrocarbons, alcohols, ethers or other oxygen compounds; they can also be the liquid or gaseous fuel (pressurized fuel xe2x80x9cCNGxe2x80x9d or liquefied fuel xe2x80x9cLPGxe2x80x9d) feeding the engine or turbine.
Patent application EP 0 540 280 A1 describes a device for reducing the emission of nitrogen oxides in the exhaust gases from internal combustion engines, said device comprising a material for the adsorption and desorption of the nitrogen oxides. According to this method, the nitrogen oxides are stored in the form of nitrates while the engine is running on a lean mixture, i.e. a hydrocarbon-lean mixture. However, the storage capacity of a trap operating according to this principle is generally impaired by the adsorption of sulfur products contained in the exhaust gases; these form sulfates, which are more stable than nitrates and poison the trap.
Furthermore, after the NoX have been trapped, it is necessary to carry out a step for desorption of the nitrogen oxides, followed by their reduction. Treatment devices are known which involve catalyzed oxidation of the carbon monoxide, CO, and the hydrocarbons, HC, contained in the exhaust gases, for example using catalysts for reducing the nitrogen oxides, called deNOx catalysts, which are active in NOx reduction over temperature ranges between 200 and 350xc2x0 C., these catalysts comprising e.g. precious metals on oxide supports, such as platinum or palladium deposited on a support of alumina, titanium oxide or zirconium oxide, or by perovskites, or over temperature ranges between 350 and 600xc2x0 C., these catalysts comprising e.g. hydrothermally stable zeolites (for example Cu-ZSM5). A device for treating the exhaust gases from a compression ignition engine, comprising a catalyst and a nitrogen oxide absorber placed in the exhaust manifold, is described for example in patents EP 0 540 280 A1 and EP 0 718 478 A1.
The material behaving like a nitrogen oxide trap must therefore be capable of adsorbing the nitrogen oxides at low temperatures, up to the temperature required for the NOx reduction catalyst to operate, the trap then assuring the desorption of the nitrogen oxides which come into contact with the deNOx catalyst at a sufficient temperature to assure the initiation of the NOx reduction reaction.
The materials forming the subject of this patent can be found in the natural state or can easily be synthesized in the laboratory [B. Durand and J. M. Paris, Compte-rendu de l""Acadxc3xa9mie des Sciences, Paris, vol. 281, series C, pp. 539-542 (1975)] [M. I. Baraton et al., Journal of Solid State Chemistry, 112, pp. 9-14 (1994)].
The invention relates to materials for removing the nitrogen oxides NO and NO2 which are present particularly in exhaust gases, for example from internal combustion engines of automotive vehicles running in a medium containing a superstoichiometric proportion of oxidizing agents, said materials being capable of adsorbing the nitrogen oxides and of desorbing the NOx when the temperature is raised and when the chemical composition of the gases is varied. The materials are mixed oxides whose structure comprises at least one metal cation A and at least one metal cation B selected from the group consisting of the elements of groups IA, IIA, IIIB, IVB, VB, VIB, VIIIB. VIII, IB, IIB, IIIA, IVA and VA, each surrounded by 6 oxygen atoms, the sum of the charges on A and B being equal to about 6, and the ratio of the cationic radius of A to the cationic radius of B being equal to 1xc2x10.4, preferably 1xc2x10.2. The cations A and B are arranged so as to form an ordered structure ABO3 of the ilmenite type.
The material according to the invention makes it possible to trap the nitrogen oxides at low temperatures and to desorb them at the temperature at which a deNOx catalyst is capable of reducing them. These materials are insensitive to the sulfur oxides and carbon oxides contained in the exhaust gases, preventing said materials from being poisoned. The materials adsorb the nitrogen oxides over a wide temperature range, whereas desorption takes place over a very narrow temperature window, affording easy management of the thermal regeneration. When desorption occurs, the previously adsorbed nitrogen oxides are emitted in puffs of high NOx concentration, which is beneficial for the kinetics of the reduction reaction of the desorbed nitrogen oxides. The kinetics of reduction of the NOx by hydrocarbons is actually of a positive order relative to the nitrogen oxide species. This material is also capable of desorbing the nitrogen oxides when the chemical composition of the gases is varied, with or without temperature variation. Said material does not have a basic oxide phase, which strongly stabilizes the nitrogen oxides and sulfur oxides in the form of nitrates and sulfates respectively. The SOx which can be adsorbed with the NOx on the material forming the subject of the invention are desorbed over a similar temperature range to that of the NOx. Prevention of the formation of stable sulfates assures less poisoning of the adsorbent material, a lower regeneration frequency and temperature and hence a longer life of the NOx trap and a gain in energy terms. According to one particular mode of carrying out the invention, association of the material claimed by the Applicant with a group VIII metal enables the adsorbed NOx to be eliminated by reduction when the gas composition is changed to a rich medium.
The present invention relates to materials for removing nitrogen oxides, said materials being oxides whose metal cations A and B are octahedrally coordinated and form a structure ABO3 of the ilmenite type. A and B are selected from elements of the periodic table in such a way that the sum of their oxidation states is equal to about +6 and so that the cations A and B are of similar size.
The material has the capability of adsorbing the NOx at low temperatures and of desorbing them at a higher temperature. It is also capable of desorbing the NOx when the ratio of reducing compounds to oxygen in the gases is increased.
More precisely, the invention relates to a method of removing the nitrogen oxides in exhaust gases, particularly from internal combustion engines of automotive vehicles, in the presence of materials having a structure ABO3 of the ilmenite type, as defined hereafter.
The adsorbent phase of the materials used in the method according to the invention has a three-dimensional structure of the ilmenite type and a stoichiometry ABO3, where:
A and B are selected from the group consisting of the elements of groups IA, IIA, IIIB, IVB, VB, VIB, VIIB, VIII, IB, IIB, IIIA, IVA and VA of the periodic table of the elements;
the O2xe2x88x92 ions occupy an octahedral position around the cations A and cations B;
the sum of the charges on the cations A and B is equal to about 6; and
the cationic radii of A and B are such that the ratio of the cationic radius of A to the cationic radius of B is equal to 1xc2x10.4, preferably 1 xc2x10.2.
The elements (A) are preferably selected from the group consisting of nickel, cobalt, iron, zinc, manganese, copper, magnesium, silver, lithium, sodium, potassium, calcium, strontium, cadmium, lead and tin, or a mixture of these elements, and particularly preferably from nickel, magnesium, zinc, cobalt, cadmium, iron and copper, or a mixture of these elements. The elements (B) are preferably selected from manganese, titanium, vanadium, niobium, silicon, zirconium, tin, chromium, antimony, bismuth and germanium, or a mixture of these elements, and particularly preferably from manganese, titanium, vanadium, niobium, silicon, tin and germanium, or a mixture of these elements.
In one particular mode of carrying out the invention, the material ABO3 also comprises at least one element (C) selected from the noble metals of group VIII of the periodic table of the elements, such as platinum, palladium, rhodium, ruthenium and iridium. The amount of metal (C) is generally between 0.05 and 5% by weight, based on the total weight of material. The material according to the invention generally has a specific surface area of between 20 and 300m2/g
There are different methods of preparing these materials. They can be synthesized by the mixing and grinding of solid inorganic precursors (oxides, carbonates etc.), followed by calcination. Depending on the compound, calcination can take place at pressures above atmospheric pressure. The materials can also be obtained by the refluxing of solutions of precursor salts (acetates, carbonates, nitrates, sulfates, chlorides etc.), drying and calcination, by the precipitation of precursor salts, the precipitate then being filtered off and calcined, or by hydrothermal synthesis, which consists in heating under autogenous pressure an aqueous solution containing the constituent components of the final material. Materials with the ilmenite structure can also be obtained by means of a decomposition reaction between a mixed oxide and a liquid corresponding to a molten salt containing the element to be incorporated into the mixed oxide. The materials obtained from these syntheses can then be modified by ion exchange with a molten salt of the cation to be exchanged. The element (C), if included, is introduced by any of the methods known to those skilled in the art: dry impregnation, excess impregnation, ion exchange, etc.
The adsorbent phases can take the form of powder, beads, pellets or extrudates; they can also be deposited or directly prepared on monolithic supports made of ceramic or metal. Advantageously, in order to increase the dispersion of the materials and hence increase their capacity to adsorb the NOx, the materials can be deposited on porous supports of large specific surface area, such as SiO2, Al2O3, TiO2, ZrO2, SiC or MgO, before being shaped (extrusion, coating etc.). These supports are generally selected from the group consisting of the following compounds: alumina (alpha, beta, delta, eta, gamma, chi or theta), silicas, aluminosilicates, zeolites, titanium oxide, zirconium oxide and finely divided carbides, for example silicon carbides, either individually or mixed. Mixed oxides or solid solutions comprising at least two of the above-mentioned oxides can be added.
However, for use on a vehicle, it is often preferable to employ rigid supports (monoliths) having a high open porosity (above 70%) in order to limit the pressure losses which might be generated by high gas flow rates, and especially the high space velocities of the exhaust gases. In fact, these pressure losses impair the engine""s performance and contribute to a lowering of the efficiency of an internal combustion engine (petrol or diesel). Furthermore, as the exhaust line is subjected to vibrations and to substantial mechanical and thermal shocks, catalysts in the form of beads, pellets or extrudates are likely to suffer damage either by attrition or by fracturing. Two techniques are employed for preparing the catalysts of the invention on monolithic supports (or substrates) made of ceramic or metal.
The first technique comprises the direct deposition of the adsorbent phase, in its final state, on the monolithic support by the coating technique known to those skilled in the art. The adsorbent phase can also be coated just after the precipitation, hydrothermal synthesis or reflux step, the final calcination step being carried out on the phase deposited directly on the monolith.
The second technique comprises first of all the deposition of the inorganic oxide on the monolithic support, then the calcination of the monolith at between 500 and 1100xc2x0 C. so that the specific surface area of this oxide is between 20 and 150 m2gxe2x88x921, and then the coating of the phase onto the monolithic substrate covered with the inorganic oxide.
The monolithic supports which can be used are made of:
either ceramic, the main components of which can be alumina, zirconia, cordierite, mullite, silica, aluminosilicates or a combination of several of these compounds;
or silicon carbide and/or nitride;
or aluminium titanate;
or a metal generally obtained from an alloy of iron, chromium and aluminium optionally doped with nickel, cobalt, cerium or yttrium.
The ceramic supports have a structure of the honeycomb type or take the form of a foam or fibres.
The metal supports can be produced by the rolling-up of corrugated strips or by the stacking of metal sheets (also corrugated) to form a honeycomb structure with straight or zigzag channels which may or may not communicate with one another. They can also be produced from interlocking, woven, braided or knitted metal fibres or filaments.
In the case of the metal supports containing aluminium in their composition, it is recommended to pretreat them at a high temperature (for example of between 700 and 1100xc2x0 C.) in order to develop a microlayer of refractory alumina on the surface. This surface microlayer, whose porosity and specific surface area are greater than those of the original metal, favour the coupling of the active phase while at the same time protecting the rest of the support against corrosion.
The amount of adsorbent phase deposited or prepared directly on the ceramic or metal support (or substrate) is generally between 20 and 300 g per liter of said support, advantageously between 50 and 200 g per liter.
The materials according to the invention therefore make it possible to adsorb and desorb the nitrogen oxides present in gases, particularly exhaust gases.
These materials are characterized in that they are capable of adsorbing the NOx at a temperature generally of between 50 and 400xc2x0 C., preferably of between 100 and 350xc2x0 C. and particularly preferably of between 150 and 300xc2x0 C. Said nitrogen oxides can be desorbed by heating to a temperature generally of between 300 and 500xc2x0 C., preferably of between 350 and 450xc2x0 C. They can also be desorbed by varying the composition of the gases, for example by punctually increasing the concentration of reducing compounds, such as hydrocarbons, hydrogen and carbon monoxide, at temperatures of between 150 and 500xc2x0 C., preferably of between 200 and 450xc2x0 C. and particularly preferably of between 250 and 400xc2x0 C. Thermally or chemically, the desorption of the nitrogen oxides can be triggered in temperature ranges in which the conventional NOx reduction catalysts are effective. Also, the thermal desorption according to the invention can take place over narrow temperature windows, generally with a width of 80xc2x0 C. Now, in the case of diesel cars, the temperature of the exhaust gases is generally between 150 and 300xc2x0 C. and rarely exceeds 500xc2x0 C. The materials used in the method according to the invention are therefore suitable for adsorption of the nitrogen oxides present in the exhaust gases from stationary engines or, in particular, automotive engines of the diesel type or of the controlled ignition type (so-called lean burn engines), but also in the gases produced by gas turbines running on gaseous or liquid fuels. These (vases are also characterized by nitrogen oxide contents of a few tens to a few thousands of parts per million (ppm) and can have comparable contents of reducing compounds (CO, H2, hydrocarbons) and sulfur, as well as substantial concentrations of oxygen (from 1 to almost 20% by volume) and water vapour. The material according to the invention can be used at HSVs of the exhaust gas (hourly space velocities, corresponding to ratios of monolith volume to gas flow rate) generally of between 500 and 150,000 hxe2x88x921, for example of between 5000 and 100,000 hxe2x88x921.
The invention further relates to the use of the materials according to the invention in a method of removing the nitrogen oxides, very particularly in a medium containing a superstoichiometric proportion of oxidizing agents. Thus the material according to the invention can be used in a method comprising:
a step for adsorption of at least part of said nitrogen oxides on an adsorbent material as defined in the present invention;
a step for desorption of the nitrogen oxides, carried out by raising the temperature or by varying the composition of the exhaust gases; and
a step for selective reduction of at least part of the nitrogen oxides to molecular nitrogen by means of reducing agents in the presence of at least one nitrogen oxide reduction catalyst.
Thus, in the nitrogen oxide reduction step, the method of removing the nitrogen oxides comprises the use of an active and selective catalyst for reducing the nitrogen oxides to molecular nitrogen by means of reducing agents in a medium containing a superstoichiometric proportion of oxidizing agents. The catalysts for reducing the nitrogen oxides to nitrogen or nitrous oxide generally comprise at least one refractory inorganic oxide and can comprise at least one zeolite selected e.g. from the following zeolites: VIFI, NU-86, NU-87 and EU-1, and generally at least one element selected from the elements of groups VIB, VIIB and VII and transition metal group IB. These catalysts can optionally contain at least one element selected from the noble metals of group VIII, for example platinum, rhodium, ruthenium, iridium and palladium, and optionally at least one element selected from the elements of alkaline earth group IIA and rare earth group IIIB. For example, the nitrogen oxide reduction catalysts comprise the following combinations: Cu-MFI, Cu-ZSM5, Fe-MFI, Fe-ZSM5, Ce-MFI, Ce-ZSM5, Pt-MFI or Pt-ZSM5.
The refractory inorganic oxide is selected from supports of the Al2O3, SiO2, ZrO2 and TiO2 type, preferably alumina.
The reducing agents are selected from CO, H2 and the hydrocarbons present in the fuel or added in the form of fresh products.
In the case where the nitrogen oxide adsorbing material according to the present invention contains at least one element (C) selected from the noble metals of group VIII of the periodic table of the elements, the method of removing the nitrogen oxides comprises:
a step for adsorption of at least part of said nitrogen oxides on the material as defined in the present invention,
a step for desorption of the nitrogen oxides; and
a step for selective reduction of at least part of the nitrogen oxides to molecular nitrogen in the presence of reducing compounds on the material as defined in the present invention.
Thus the reduction of the nitrogen oxides to nitrogen or nitrous oxide can take place directly on the adsorbent material according to the invention, which makes it possible simultaneously to trap the nitrogen oxides and to desorb and reduce said nitrogen oxides.